Solid state imaging devices, including charge coupled devices (CCD), complementary metal oxide semiconductor (CMOS) imaging devices and others, have been used in photo imaging applications. A solid state imaging device includes a focal plane array of pixel cells, or pixels as an image sensor. Each cell includes a photosensor, which may be a photogate, a photoconductor, a photodiode, or other photosensor having a doped region for accumulating photo-generated charge. For CMOS imaging devices, each pixel cell has a charge storage region, formed on or in the substrate, which is connected to the gate of an output transistor that is part of a readout circuit. The charge storage region may be constructed as a floating diffusion region. In some CMOS imaging devices, each pixel cell may further include at least one electronic device, such as a transistor for transferring charge from the photosensor to the storage region and one device, also typically a transistor, for resetting the storage region to a predetermined charge level.
One problem experienced in film and solid state cameras is lens shading. Lens shading can cause pixel cells in a pixel array located farther away from the center of the pixel array to have a lower pixel signal value when compared to pixel cells located closer to the center of the pixel array, even when all pixel cells are exposed to the same illuminant condition. Optical vignetting and pixel vignetting are two sources of lens shading. Optical vignetting is the fall-off in irradiance inherent in optical lens systems and is more pronounced with wide angle lenses and lenses using wide open apertures. Pixel vignetting, on the other hand, is the fall-off in irradiance inherent in photosensors and is affected by many factors such as, for example, microlens placement, photosensor layout and depth of the photon well.
Variations in a pixel value caused by lens shading can be measured and the pixel value can be adjusted to compensate for the lens shading. For example, lens shading can be adjusted using a set of positional gain adjustment values, which modify pixel values in post-image capture processing. Positional gain adjustments across the pixel array can be provided as digital gain values, one corresponding to each of the pixels. It may happen that the further away a pixel is from the center of the pixel array, the more gain is needed to be applied to the pixel value. The set of digital gain values for the entire pixel array forms a gain adjustment surface.
Lens shading correction for color image sensors may be defined for each of a plurality of color channels in order to correct for lens shading variations across color channels. For these color image sensors, the gain adjustment surface is applied to the pixels of the corresponding color channel during post-image capture processing to correct for variations in pixel value due to the spatial location of the pixels in the pixel array. Monochrome image sensors, on the other hand, apply a single gain adjustment surface to all pixels of a pixel array. Likewise, color image sensors may choose to use a single gain adjustment surface across all color channels.
Since lens shading correction applies a digital gain at every pixel location, the digital gain values need to be either pre-stored or digitally computed by mimicking the desired pixel gain surface. In practice, the digital gain values are computed from an expression that approximates the desired pixel gain surface since the number of parameters needed to generate an approximate surface is generally significantly lower than the numbers of parameters needed to store the digital gain values for every pixel location. Some image sensors have built-in lens shading operation on-chip, while other image sensors rely on a separate image processing imaging chip for this operation.
It has been found that the lens shading properties of an image sensor can vary significantly with the spectral content of an imaged scene. When a gain adjustment surface is calculated for a specific color channel/camera/lens/IR-output filter, etc. combination, it is generally applied to all captured images from an imaging device having that combination. This does not present a particular problem when the captured image is taken under the same illuminant condition used to calibrate the gain adjustment surface. Lens shading, however, can vary significantly from one illuminate type to another type.
In FIG. 1, a plot of red pixel values along the center row of an image sensor is shown prior to lens shading correction. The plot illustrates the significant variation in lens shading under three illuminant conditions that are well-known to persons skilled in the art of image processing: D65, cool white fluorescent (CWF), and type A flat-fields. Application of a gain adjustment surface calibrated for a particular illuminant type, e.g., D65 flat-field, to an image captured under a different illuminant type, e.g., type A flat-field, can result in undesirable color shading errors.
Accordingly, methods and systems providing lens shading correction for pixel values of images captured under varying illuminant conditions are desirable.